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Arris Motorola SB6121 Review

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PROS / This cable modem works with just about every major cable internet provider.

CONS / There isn't a power button, which means you'll need to unplug the modem to power it down.

 VERDICT / The Arris Motorola SB6121 is a great cable modem that will work well for most people. If you have a very fast internet connection, you may want to look elsewhere.

The Arris Motorola SB6121 is a great cable modem that sports compatibility with a wide variety of cable internet providers. In fact, the SB6121 works with nine of the 10 largest cable internet providers in the United States. In addition to its compatibility, this cable modem has good performance marks and adheres to all of the standards we look for in the best cable modems. Because of its well-balanced approach, the SB6121 earns our Top Ten Reviews Silver Award.

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The SB6121 is similar to the SB6141, the number one product on our lineup, in almost every way. The only real difference is that the SB6121 has 4 x 4 channel bonding instead of the SB6141's 8 x 4 channel bonding.

  1. What percentage of the 10 largest ISPs this modem works with.
    Higher is Better
  2. 2 Arris Motorola SB6121
  3. 95%
  4. 85%
  5. Category Average

ISP Compatibility

If you're subscribed to internet service from one of the 10 largest cable internet providers in the United States, then chances are the SB6121 will work just fine for you. Like the SB6141, this cable modem works with nine of the 10 largest cable internet providers. This includes the four largest providers: Comcast, Time Warner Cable, Charter and Cox. The major advantage here, other than working with your current service, is that if you move in the future, the SB6121 will likely work with your future cable internet provider as well.


The SB6121 has a maximum theoretical download speed of 172 Mbps and an upload speed of 131 Mbps. However, after you figure in network overhead and other factors, like network congestion, you're looking at download speeds closer to 100 Mbps than 170 Mbps. This is still fast enough for most people, as most people don't have access to internet speeds of more than 100 Mbps. But if you do subscribe to your internet provider's highest speed tiers, then you might want to look at cable modems that are capable of higher speeds.

We were impressed with the SB6121's average operating temperature. At 87.7 degrees Fahrenheit, the SB6121 has one of the lowest operating temperatures we measured in our testing. This is good news, because it means that the SB6121 efficiently handles the power it draws from your wall outlet. A lower operating temperature can also mean a longer life for the SB6121. That's actually true for just about any electronic device. You want them to run as cool as possible.


DOCSIS (Data Over Cable Service Interface Specification) is an interoperability standard that is maintained by an industry trade organization called CableLabs. The Motorola SB6121 uses the latest version, DOCSIS 3.0, and can connect at speeds four times faster than the DOCSIS 2.0 standard allows. Of course, for the best results, your internet provider's service needs to be running DOCSIS 3.0 as well.

The biggest difference between the SB6121 and its bigger sibling, the SB6141, is that the SB6121 only has 4 x 4 channel bonding. That means you get four channels for downloads and four for uploads. The SB6141 has eight channels for downloads. Essentially, the SB6121 gets half the download speed you would get from the SB6141. However, that won't matter much unless you have an internet connection of more than 100 Mbps.

CableLabs has certified the SB6121 as a DOCSIS 3.0 modem, which means that it should work well with any DOCSIS 3.0 infrastructure. UL also certified the SB6121, but it looks at some other things. UL focuses on ensuring that consumer products are safe to use. With the UL certification, we know that the SB6121 adheres to industry regulations and safety standards.


In addition to supporting the faster DOCSIS technology, this modem supports a Gigabit Ethernet connection between the computer and the cable modem, which is much faster than the Fast Ethernet connection found on a typical DOCSIS 2.0 cable modem.

Unfortunately, the SB6121 doesn't have a reset button or a power button. In order to power down the device, you have to unplug it. While this isn't really a big hassle, it's still an inelegant solution compared to the modems that do have a reset button or power button.

Help & Support

Arris provides a two-year warranty and 24/7 live technical support. A data sheet with specifications plus a user guide is online. You also get a pocket guide with LED diagnostics and a useful flowchart that explains the steps for setting up the cable modem in the modem's packaging.


The Arris Motorola SB6121 is a great option for most people. It's edged out of the top spot by the SB6141, which has superior download capability. Even though the SB6121 may not be the very best cable modem on our lineup, it still has much to recommend it. The high internet provider compatibility means you can take the modem with you when you move. The low operating temperature means that the SB6121 will likely last for several years. This is the cable modem to get if you don't need very fast download speeds.

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